Due to the increasing number of electronic components on the one hand and due to the networking thereof by way of bus systems in motor vehicles on the other hand, electromagnetic compatibility (EMC) has gained considerably in relevance in the recent years. In principle this means that an electronic component must be insensitive under all specified operating conditions to influences acting thereon from the exterior, it may not influence other electrical systems and during operation thereof it must permit uninterrupted radio reception—both in the automobile itself and also in the environment thereof. The requirements which are to be met in relation to EMC are laid down in the so-called ‘Automotive EMC Directive’.
Even products which are not directly confronted with the EMC problems are in the meantime equipped with complex integrated electronic circuits like for example interior heaters, diesel quick-start systems or other switched high-current consumers.
The development of diesel engines has made rapid advances in recent years. Marked improvements have been achieved in terms of emission, consumption, torque production and power density by virtue of new developed components, systems and by virtue of engine management. A glow system (instant start system (ISS)) comprises an electronic glow plug control device and power-optimised glow plugs. Power semiconductors are used in the control device as switches for actuation of the glow plugs, replacing the electromagnetic relay used for that purpose. Each glow plug can be individually actuated in that case.
With electronically controlled glow plugs, temperature behaviour and power consumption are adapted to the glow requirement of the engine by the control device over a wide range. The power consumption is modified by clock control (for example pulse width modulation) of the glow plug current by means of the power semiconductor. In that respect it is possible to achieve a high level of efficiency for the system so that scarcely more than the power required by the glow plug is taken from the on-board circuit system. As with ISS each glow plug is actuated by a separate power semiconductor the current can be separately monitored in each glow plug circuit. That permits individual diagnosis at each plug.
FIG. 2 shows time graphs of pulsed individual load currents and a resulting overall current in a conventionally controlled glow system, wherein pulse-form load currents i1 through i3 are fed to the glow plugs and by virtue of their finite edge steepness result in collapses in the overall current iG with a period duration Δt.
Due to the clock control of the high glow plug currents at frequencies of 30 through 100 Hz, detrimental rapid changes in voltage or current thus occur, which can lead to disturbances in respect of EMC.